Field of the Invention
The present invention relates to mobile communication.
Related Art
In 3GPP in which technical standards for mobile communication systems are established, in order to handle 4th generation communication and several related forums and new technologies, research on Long Term Evolution/System Architecture Evolution (LTE/SAE) technology has started as part of efforts to optimize and improve the performance of 3GPP technologies from the end of the year 2004
SAE that has been performed based on 3GPP SA WG2 is research regarding network technology that aims to determine the structure of a network and to support mobility between heterogeneous networks in line with an LTE task of a 3GPP TSG RAN and is one of recent important standardization issues of 3GPP. SAE is a task for developing a 3GPP system into a system that supports various radio access technologies based on an IP, and the task has been carried out for the purpose of an optimized packet-based system which minimizes transmission delay with a more improved data transmission capability.
An Evolved Packet System (EPS) higher level reference model defined in 3GPP SA WG2 includes a non-roaming case and roaming cases having various scenarios, and for details therefor, reference can be made to 3GPP standard documents TS 23.401 and TS 23,402, A network configuration of FIG. 1 has been briefly reconfigured from the EPS higher level reference model.
FIG. 1 shows the configuration of an evolved mobile communication network.
An Evolved Packet Core (EPC) may include various elements. FIG. 1 illustrates a Serving Gateway (S-GW) 52, a Packet Data Network Gateway (PDN GW) 53, a Mobility Management Entity (MME) 51, a Serving General Packet Radio Service (GPRS) Supporting Node (SGSN), and an enhanced Packet Data Gateway (ePDG) that correspond to some of the various elements.
The S-GW 52 is an element that operates at a boundary point between a Radio Access Network (RAN) and a core network and has a function of maintaining a data path between an eNodeB 22 and the PDN GW 53. Furthermore, if a terminal (or User Equipment (UE) moves in a region in which service is provided by the eNodeB 22, the S-GW 52 plays a role of a local mobility anchor point. That is, for mobility within an E-UTRAN (i.e., a Universal Mobile Telecommunications System (Evolved-UMTS) Terrestrial Radio Access Network defined after 3GPP release-8), packets can be routed through the S-GW 52. Furthermore, the S-GW 52 may play a role of an anchor point for mobility with another 3GPP network (i.e., a RAN defined prior to 3GPP release-8, for example, a UTRAN or Global System for Mobile communication (GSM) (GERAN)/Enhanced Data rates for Global Evolution (EDGE) Radio Access Network).
The PDN GW (or P-GW) 53 corresponds to the termination point of a data interface toward a packet data network. The PDN GW 53 can support policy enforcement features, packet filtering, charging support, etc. Furthermore, the PDN GW (or P-GW) 53 can play a role of an anchor point for mobility management with a 3GPP network and a non-3GPP network (e.g., an unreliable network, such as an Interworking Wireless Local Area Network (I-WLAN), a Code Division Multiple Access (CDMA) network, or a reliable network, such as WiMax).
In the network configuration of FIG. 1, the S-GW 52 and the PDN GW 53 have been illustrated as being separate gateways, but the two gateways may be implemented in accordance with a single gateway configuration option.
The MME 51 is an element for performing the access of a terminal to a network connection and signaling and control functions for supporting the allocation, tracking, paging, roaming, handover, etc. of network resources. The MME 51 controls control plane functions related to subscribers and session management. The MME 51 manages numerous eNodeBs 22 and performs conventional signaling for selecting a gateway for handover to another 2G/3G networks. Furthermore, the MME 51 performs functions, such as security procedures, terminal-to-network session handling, and idle terminal location management.
The SGSN handles all packet data, such as a user's mobility management and authentication for different access 3GPP networks (e.g., a GPRS network and an UTRAN/GERAN).
The ePDG plays a role of a security node for an unreliable non-3GPP network (e.g., an I-WLAN and a Wi-Fi hotspot).
As described with reference to FIG. 1, a terminal (or UE) having an IP capability can access an IP service network (e.g., IMS), provided by a service provider (i.e., an operator), via various elements within an EPC based on non-3GPP access as well as based on 3GPP access.
Furthermore, FIG. 1 shows various reference points (e.g., S1-U and S1-MME). In a 3GPP system, a conceptual link that connects two functions that are present in the different function entities of an E-UTRAN and an EPC is called a reference point. Table 1 below defines reference points shown in FIG. 1. In addition to the reference points shown in the example of Table 1, various reference points may be present depending on a network configuration.
TABLE 1REFERENCEPOINTDESCRIPTIONS1-MMEA reference point for a control plane protocol between the E-UTRAN andthe MMES1-UA reference point between the E-UTRAN and the S-GW for path switchingbetween eNodeBs during handover and user plane tunneling per bearerS3A reference point between the MME and the SGSN that provides theexchange of pieces of user and bearer information for mobility between3GPP access networks in idle and/or activation state. This reference pointcan be used intra-PLMN or inter-PLMN (e.g. in the case of Inter-PLMNHO).S4A reference point between the SGW and the SGSN that provides relatedcontrol and mobility support between the 3GPP anchor functions of aGPRS core and the S-GW. Furthermore, if a direct tunnel is notestablished, the reference point provides user plane tunneling.S5A reference point that provides user plane tunneling and tunnelmanagement between the S-GW and the PDN GW. The reference point isused for S-GW relocation due to UE mobility and if the S-GW needs toconnect to a non-collocated PDN GW for required PDN connectivityS11A reference point between the MME and the S-GWSGiA reference point between the PDN GW and the PDN. The PDN may bea public or private PDN external to an operator or may be an intra-operatorPDN, e.g., for the providing of IMS services. This reference pointcorresponds to Gi for 3GPP access.RxA reference point between PCRF and AF (Application Function), AF canbe P-CSCF of IMS network
Among the reference points shown in FIG. 1, S2a and S2b correspond to a Non-3GPP interface. S2a is a reference point that provides the user plane with the relevant control and mobility support between trusted Non-3GPP access and PDN GW. S2b is a reference point providing the user plane with the associated control and mobility support between the ePDG and the PDN GW.
FIG. 2 is an exemplary diagram showing the architecture of a common E-UTRAN and a common EPC.
As shown in FIG. 2, the eNodeB 20 can perform functions, such as routing to a gateway while RRC connection is activated, the scheduling and transmission of a paging message, the scheduling and transmission of a broadcast channel (BCH), the dynamic allocation of resources to UE in uplink and downlink, a configuration and providing for the measurement of the eNodeB 20, control of a radio bearer, radio admission control, and connection mobility control. The EPC can perform functions, such as the generation of paging, the management of an LTE IDLE state, the ciphering of a user plane, control of an EPS bearer, the ciphering of NAS signaling, and integrity protection.
FIG. 3 is an exemplary diagram showing the structure of a radio interface protocol in a control plane between UE and an eNodeB, and FIG. 4 is another exemplary diagram showing the structure of a radio interface protocol in a control plane between UE and an eNodeB.
The radio interface protocol is based on a 3GPP radio access network standard. The radio interface protocol includes a physical layer, a data link layer, and a network layer horizontally, and it is divided into a user plane for the transmission of information and a control plane for the transfer of a control signal (or signaling).
The protocol layers may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of the Open System Interconnection (OSI) reference model that is widely known in communication systems.
The layers of the radio protocol of the control plane shown in FIG. 3 and the radio protocol in the user plane of FIG. 4 are described below.
The physical layer PHY, that is, the first layer, provides information transfer service using physical channels. The PHY layer is connected to a Medium Access Control (MAC) layer placed in a higher layer through a transport channel, and data is transferred between the MAC layer and the PHY layer through the transport channel. Furthermore, data is transferred between different PHY layers, that is, PHY layers on the sender side and the receiver side, through the PHY layer.
A physical channel is made up of multiple subframes on a time axis and multiple subcarriers on a frequency axis. Here, one subframe is made up of a plurality of symbols and a plurality of subcarriers on the time axis. One subframe is made up of a plurality of resource blocks, and one resource block is made up of a plurality of symbols and a plurality of subcarriers. A Transmission Time Interval (TTI), that is, a unit time during which data is transmitted, is 1 ms corresponding to one subframe.
In accordance with 3GPP LTE, physical channels that are present in the physical layer of the sender side and the receiver side can be divided into a Physical Downlink Shared Channel (PDSCH) and a Physical Uplink Shared Channel (PUSCH), that is, data channels, and a Physical Downlink Control Channel (PDCCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel (PHICH), and a Physical Uplink Control Channel (PUCCH), that is, control channels.
A PCFICH that is transmitted in the first OFDM symbol of a subframe carries a Control Format Indicator (CFI) regarding the number of OFDM symbols (i.e., the size of a control region) used to send control channels within the subframe. A wireless device first receives a CFI on a PCFICH and then monitors PDCCHs.
Unlike a PDCCH, a PCFICH is transmitted through the fixed PCFICH resources of a subframe without using blind decoding.
A PHICH carries positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signals for an uplink (UL) Hybrid Automatic Repeat reQuest (HARQ). ACK/NACK signals for UL data on a PUSCH that is transmitted by a wireless device are transmitted on a PHICH.
A Physical Broadcast Channel (PBCH) is transmitted in four former OFDM symbols of the second slot of the first subframe of a radio frame. The PBCH carries system information that is essential for a wireless device to communicate with an eNodeB, and system information transmitted through a PBCH is called a Master Information Block (MIB). In contrast, system information transmitted on a PDSCH indicated by a PDCCH is called a System Information Block (SIB).
A PDCCH can carry the resource allocation and transport format of a downlink-shared channel (DL-SCH), information about the resource allocation of an uplink shared channel (UL-SCH), paging information for a PCH, system information for a DL-SCH, the resource allocation of an upper layer control message transmitted on a PDSCH, such as a random access response, a set of transmit power control commands for pieces of UE within a specific UE group, and the activation of a Voice over Internet Protocol (VoIP). A plurality of PDCCHs can be transmitted within the control region, and UE can monitor a plurality of PDCCHs. A PDCCH is transmitted on one Control Channel Element (CCE) or an aggregation of multiple contiguous CCEs. A CCE is a logical allocation unit used to provide a PDCCH with a coding rate according to the state of a radio channel. A CCE corresponds to a plurality of resource element groups. The format of a PDCCH and the number of bits of a possible PDCCH are determined by a relationship between the number of CCEs and a coding rate provided by CCEs.
Control information transmitted through a PDCCH is called Downlink Control Information (DCI). DCI can include the resource allocation of a PDSCH (also called a downlink (DL) grant)), the resource allocation of a PUSCH (also called an uplink (UL) grant), a set of transmit power control commands for pieces of UE within a specific UE group, and/or the activation of a Voice over Internet Protocol (VoIP).
Several layers are present in the second layer. First, a Medium Access Control (MAC) layer functions to map various logical channels to various transport channels and also plays a role of logical channel multiplexing for mapping multiple logical channels to one transport channel. The MAC layer is connected to a Radio Link Control (RLC) layer, that is, a higher layer, through a logical channel. The logical channel is basically divided into a control channel through which information of the control plane is transmitted and a traffic channel through which information of the user plane is transmitted depending on the type of transmitted information.
The RLC layer of the second layer functions to control a data size that is suitable for sending, by a lower layer, data received from a higher layer in a radio section by segmenting and concatenating the data. Furthermore, in order to guarantee various types of QoS required by radio bearers, the RLC layer provides three types of operation modes: a Transparent Mode (TM), an Un-acknowledged Mode (UM), and an Acknowledged Mode (AM). In particular, AM RLC performs a retransmission function through an Automatic Repeat and Request (ARQ) function for reliable data transmission.
The Packet Data Convergence Protocol (PDCP) layer of the second layer performs a header compression function for reducing the size of an IP packet header containing control information that is relatively large in size and unnecessary in order to efficiently send an IP packet, such as IPv4 or IPv6, in a radio section having a small bandwidth when sending the IP packet. Accordingly, transmission efficiency of the radio section can be increased because only essential information is transmitted in the header part of data. Furthermore, in an LTE system, the PDCP layer also performs a security function. The security function includes ciphering for preventing the interception of data by a third party and integrity protection for preventing the manipulation of data by a third party.
A Radio Resource Control (RRC) layer at the highest place of the third layer is defined only in the control plane and is responsible for control of logical channels, transport channels, and physical channels in relation to the configuration, re-configuration, and release of Radio Bearers (RBs). Here, the RB means service provided by the second layer in order to transfer data between UE and an E-UTRAN.
If an RRC connection is present between the RRC layer of UE and the RRC layer of a wireless network, the UE is in an RRC_CONNECTED state. If not, the UE is in an RRC_IDLE state.
An RRC state and an RRC connection method of UE are described below. The RRC state means whether or not the RRC layer of UE has been logically connected to the RRC layer of an E-UTRAN. If the RRC layer of UE is logically connected to the RRC layer of an E-UTRAN, it is called the RRC_CONNECTED state. If the RRC layer of UE is not logically connected to the RRC layer of an E-UTRAN, it is called the RRC_IDLE state. Since UE in the RRC_CONNECTED state has an RRC connection, an E-UTRAN can check the existence of the UE in a cell unit, and thus control the UE effectively. In contrast, if UE is in the RRC_IDLE state, an E-UTRAN cannot check the existence of the UE, and a core network is managed in a Tracking Area (TA) unit, that is, an area unit greater than a cell. That is, only the existence of UE in the RRC_IDLE state is checked in an area unit greater than a cell. In such a case, the UE needs to shift to the RRC_CONNECTED state in order to be provided with common mobile communication service, such as voice or data. Each TA is classified through Tracking Area Identity (TAI). UE can configure TAI through Tracking Area Code (TAC), that is, information broadcasted by a cell.
When a user first turns on the power of UE, the UE first searches for a proper cell, establishes an RRC connection in the corresponding cell, and registers information about the UE with a core network. Thereafter, the UE stays in the RRC_IDLE state. The UE in the RRC_IDLE state (re)selects a cell if necessary and checks system information or paging information. This process is called camp on. When the UE in the RRC_IDLE state needs to establish an RRC connection, the UE establishes an RRC connection with the RRC layer of an E-UTRAN through an RRC connection procedure and shifts to the RRC_CONNECTED state. A case where the UE in the RRC_IDLE state needs to establish with an RRC connection includes multiple cases. The multiple cases may include, for example, a case where UL data needs to be transmitted for a reason, such as a call attempt made by a user and a case where a response message needs to be transmitted in response to a paging message received from an E-UTRAN.
A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management.
The NAS layer shown in FIG. 3 is described in detail below.
Evolved Session Management (ESM) belonging to the NAS layer performs functions, such as the management of default bearers and the management of dedicated bearers, and ESM is responsible for control that is necessary for UE to use PS service from a network. Default bearer resources are characterized in that they are allocated by a network when UE first accesses a specific Packet Data Network (PDN) or accesses a network. Here, the network allocates an IP address available for UE so that the UE can use data service and the QoS of a default bearer. LTE supports two types of bearers: a bearer having Guaranteed Bit Rate (GBR) QoS characteristic that guarantees a specific bandwidth for the transmission and reception of data and a non-GBR bearer having the best effort QoS characteristic without guaranteeing a bandwidth. A default bearer is assigned a non-GBR bearer, and a dedicated bearer may be assigned a bearer having a GBR or non-GBR QoS characteristic.
In a network, a bearer assigned to UE is called an Evolved Packet Service (EPS) bearer. When assigning an EPS bearer, a network assigns one ID. This is called an EPS bearer ID. One EPS bearer has QoS characteristics of a Maximum Bit Rate (MBR) and a Guaranteed Bit Rate (GBR) or an Aggregated Maximum Bit Rate (AMBR).
FIG. 5 illustrates a connection process in a radio resource control (RRC) layer.
FIG. 5 shows an RRC state depending on whether there is an RRC connection. The RRC state denotes whether the entity of the RRC layer of UE 10 is in logical connection with the entity of the RRC layer of eNodeB 20, and if yes, it is referred to as RRC connected state, and if no as RRC idle state.
In the connected state, UE 10 has an RRC connection, and thus, the E-UTRAN may grasp the presence of the UE on a cell basis and may thus effectively control UE 10. In contrast, UE 10 in the idle state cannot grasp eNodeB 20 and is managed by a core network on the basis of a tracking area that is larger than a cell. The tracking area is a set of cells. That is, UE 10 in the idle state is grasped for its presence only on a larger area basis, and the UE should switch to the connected state to receive a typical mobile communication service such as voice or data service.
When the user turns on UE 10, UE 10 searches for a proper cell and stays in idle state in the cell. UE 10, when required, establishes an RRC connection with the RRC layer of eNodeB 20 through an RRC connection procedure and transits to the RRC connected state.
There are a number of situations where the UE staying in the idle state needs to establish an RRC connection, for example, when the user attempts to call or when uplink data transmission is needed, or when transmitting a message responsive to reception of a paging message from the EUTRAN.
In order for the idle UE 10 to be RRC connected with eNodeB 20, UE 10 needs to perform the RRC connection procedure as described above. The RRC connection procedure generally comes with the process in which UE 10 transmits an RRC connection request message to eNodeB 20, the process in which eNodeB 20 transmits an RRC connection setup message to UE 10, and the process in which UE 10 transmits an RRC connection setup complete message to eNodeB 20. The processes are described in further detail with reference to FIG. 6.
1) The idle UE 10, when attempting to establish an RRC connection, e.g., for attempting to call or transmit data or responding to paging from eNodeB 20, sends an RRC connection request message to eNodeB 20.
2) When receiving the RRC connection message from UE 10, eNodeB 20 accepts the RRC connection request from UE 10 if there are enough radio resources, and eNodeB 20 sends a response message, RRC connection setup message, to UE 10.
3) When receiving the RRC connection setup message, UE 10 transmits an RRC connection setup complete message to eNodeB 20. If UE 10 successfully transmits the RRC connection setup message, UE 10 happens to establish an RRC connection with eNodeB 20 and switches to the RRC connected state.
FIG. 6 shows a connection between an EPC and an IP multimedia subsystem (IMS).
Referring to FIG. 6, the EPC includes an MME 51, an S-GW 52, a P-GW 53a to be coupled to the IMS, a P-GW 53b to be coupled to the Internet, and a policy and charging rule function (PCRF) 58 to be coupled to the P-GW 53a. 
A network technology which enables up to a wireless terminal to perform packet switching (PS) based on an Internet protocol (IP) is proposed to connect both wired/wireless terminals through all-IPs.
A network based on the IMS includes a call session control function (CSCF) for control signaling, registration, and cession processing and a session and interconnection border control function (IBCF) 62. The CSCF may include a proxy-CSCF (P-CSCF) 61 and an S-CSCF (Serving-CSCF) 63. In addition, the CSCF may include an interrogating-CSCF (I-CSCF). The P-CSCF 61 acts as a first access point for a user equipment (UE) in the IMS-based network. In addition, the S-CSCF 63 processes a session in the IMS network. That is, the S-SCSF 63 is an entity which is in charge of routing signaling, and routes the session in the IMS network. In addition, the I-CSCF acts as an access point with respect to another entity within the IMS network.
An IP-based session is controlled by a session initiation protocol (SIP) under the IMS. The SIP is a protocol for controlling the session. The SIP is a signaling protocol which specifies a procedure for finding locations by identifying UEs to be communicated, generating a multimedia service session between the UEs, and deleting and changing the generated session. The SIP uses an SIP uniform resource identifier (URI) similar to an e-mail address to distinguish each user, so that a service can be provided without being dependent on an Internet protocol (IP) address. The SIP message is a control message, but is transmitted between the UE and the IMS network through an EPC user plane.
Referring to FIG. 6, the first P-GW 53a of the EPC is coupled to the P-CSCF 61 of the IMS, the P-CSCF 61 is coupled to the IBCF 62, and the IBCF 62 is coupled to the S-CSCF 63.
In addition, the second P-GW 53b of the EPC is coupled to a network of the Internet service operator.
Hereinafter, an initial access procedure of the UE 10 is described.
According to the initial access procedure, the EPC may allocate a default bearer to the UE 10, and may register the UE 10. In addition, the UE 10 may be allocated an IP address to use an IMS network from the PGW 53, and may obtain an address of the P-CSCF 61 to register to an IMS network.
FIG. 7 is an exemplary signal flow diagram showing an initial access procedure of a UE.
Referring to FIG. 7, for an initial access, the UE 10 which has been powered on configures an RRC connection with the eNodeB 20 as described with reference to FIG. 5 (S101).
After the RRC connection with the eNodeB 20 is established, the UE 10 transmits an attach request message to the MME 51 (S103). A PDN connectivity request message may be included in the attach request message. In this case, the UE 10 may request for an address of the P-CSCF 61 by using a protocol configuration option (PCO) field.
The MME 51 performs an authentication and security setup procedure for the UE 10 in association with the HSS 54 (S105). In the authentication procedure, the MME 51 obtains an authentication vector for a subscriber from the HSS 54, and thereafter performs mutual authentication with respect to the UE 10 by using the authentication vector. When the authentication procedure is complete, the MME 51 establishes a security key for the message security setup between the UE 10 and the MME 51.
The MME 51 performs a location registration procedure to inform the HSS 54 that the UE 10 is located in a region managed by the MME 51, and receives a user profile (S 107). The location registration procedure may be performed by using a diameter protocol on an S6a interface. In addition, the user profile received by the MME 51 may include an access point name (APN), a P-GW identifier, a quality of service (QoS) profile, or the like.
The MME 51 selects the P-GW 53, and transmits a create session request message to the selected P-GW 53 (S109). The create session request message may include the user profile and the PCO field requesting an address of the P-CSCF 61. The create session request message transmitted by the MME 51 may be delivered to the P-GW 53 via the S-GW 52.
The P-GW 53 allocates the IP of the UE 10, and selects an address list of the P-CSCFs 61 which can be used by the UE among a plurality of P-CSCFs 61 according to the PCO field. Optionally, the P-GW 53 transmits an ‘indication of IP-CAN session establishment’ message to the PCRF 58 (S111). In addition, the P-GW 53 receives an ‘acknowledge of IP-CON session establishment’ message from the PCRF 58 (S113). The ‘acknowledge of IP-CON session establishment’ message may include a policy of a service to be provided to the UE 10.
The P-GW 53 transmits a create session response message to the MME 51 (S115). The create session response message may include an IP allocated to the UE 10 and the address list of the P-CSCF 61. The create session response message transmitted by the P-GW 53 may be transmitted to the MME 51 via the S-GW 52.
The MME 51 transmits an attach accept message including an initial context setup request message to the eNodeB 20. In addition, the eNodeB 20 transmits to the UE an access accept message including an RRC connection reconfiguration message and an activate default EPS bearer context request message (S117).
In step S119, the UE 10 transmits an RRC connection reconfiguration complete message to the eNodeB 20 in response to reception of the RRC connection reconfiguration message (S119). The eNodeB 20 transmits an initial context setup response message to the MME 51 in response to reception of the initial context setup request message (S121).
The MME 51 transmits a modify bearer request message to the S-GW 52 in response to reception of the initial context setup response message (S123). The bearer modify request message may include an EPS bearer identifier, an eNodeB address, a handover indication, or the like. The S-GW 52 transmits a modify bearer response message to the MME 51 in response to reception of the modify bearer response message (S 125).
Hereinafter, an IMS initial registration procedure of the UE 10 will be described.
FIG. 8 is an exemplary signal flow diagram showing an IMS initial registration procedure.
Referring to FIG. 8, the UE 10 transmits a register message requesting for a registration to the P-CSCF 61 (S 201). The UE 10 may transmit a register message by using an address of the P-CSCF 61, which is identified through the activate default EPS bearer context request message.
The P-CSCF 61 delivers the register message received from the UE 10 to the I-CSCF 64 by using an address of the I-CSCF 64, which is obtained through a domain name system (DNS) query procedure (S203).
The I-CSCF 64 transmits a user authorization request (UAR) message to the HSS 54 (S205). Since there is no S-CSCF 63 allocated to the UE 10, the HSS 54 transmits to the I-CSCF 64 a user authorization answer (UAA) message including capability information of the UE 10 (S207). The capability information is information in which capability to be provided to the UE 10 is organized with an attribute value pair (AVP).
The I-CSCF 64 selects one S-CSCF 63 on the basis of the received capability information, and transmits a register message to the selected S-CSCF 63 (S209).
The S-CSCF 63 transmits a multimedia authentication request (MAR) message to the HSS 54 to request for authentication information regarding the UE 10 (S211). Since there is no authentication information regarding the UE 10 due to the IMS initial registration, the HSS 54 transmits a multimedia authentication answer (MAA) message for informing that the authentication information is required to the S-CSCF 63 (S213).
The S-CSCF 63 transmits a 401 unauthorized message for requesting for the authentication information to the UE 10 (S215). The 401 unauthorized message may include an authentication vector received from the HSS, a symmetric key designated by the S-CSCF 63, and an authentication algorithm. The 401 unauthorized message may be delivered to the UE 10 via the I-CSCF 64 and the P-CSCF 61.
The UE 10 generates authentication data by using the received authentication vector, symmetric key, and authentication algorithm, and transmits the register message including the generated authentication data to the P-CSCF 61 (S217). The P-CSCF 61 delivers the received register message to the I-CSCF 64 (S219).
The I-CSCF 64 transmits the UAR message to the HSS 54 (S221). Since the S-CSCF 63 allocated to the UE 10 exists, the HSS 54 transmits the UAA message including the identification information of the allocated S-CSCF 63 to the I-CSCF 64 since (S223). The I-CSCF 64 transmits the register message to the S-CSCF 63 (S225).
The S-CSCF 63 authenticates the UE 10 by comparing authentication data included in the register message and authentication information transmitted by the S-CSCF 63, and transmits a server assignment request (SAR) message to the HSS (S227). The HSS 54 transmits to the S-CSCF 63 a server assignment answer (SAA) message including a service profile for the UE 10 (S229).
The S-CSCF 63 transmits to the UE 10 a 200 OK message notifying that the registration is complete, thereby completing the registration procedure (S231). The 200 OK message may be delivered to the UE 100 via the I-CSCF 64 and the P-CSCF 61.
FIG. 9 is an exemplary diagram showing a roaming scheme of voice over LTE (VoLTE).
As can be seen with reference to FIG. 9, the roaming scheme of VoLTE includes a home routed (HR) scheme and a local breakout (LBO) scheme.
According to the LBO scheme, IMS signaling transmitted from a UE is delivered to an S-CSCF in a home PLMN (H-PLMN) via an S-GW/P-GW/P-CSCF in a visited public land mobile network (V-PLMN).
In the HR scheme, the IMS signaling is delivered to the S-CSCF after passing through a P-GW/P-CSCF in the H-PLMH via the S-GW in the V-PLMN.
FIG. 10 is an exemplary signal flow diagram showing an IMS registration procedure of a UE roaming in a visited network through an HR scheme.
Hereinafter, when the IMS registration procedure based on the HR scheme of the UE 10 roaming in the visited network through the HR scheme is described, the duplicated description of FIG. 8 will be omitted.
Referring to FIG. 10, the UE 10 roaming in a visited network (or V-PLMN) transmits a register message to the S-GW 52b of the visited network via an eNB. The S-GW 52b of the visited network transmits the received register message to the P-GW 53a of a home network, and the P-GW 53a transmits the received register message to the P-CSCF 61a (S301). That is, the UE 10 transmits the register message to not a control plane but a user plane.
The P-CSCF 61a subscribes a network identifier (or PLMN-ID) change notification to the PCRF 58a (S303). In this case, the PLMN-ID change notification may be subscribed through an Rx interface. The Rx interface is an interface for exchanging information between the P-CSCF 61a of an IMS network and the PCRF 58a of an EPC network.
The PCRF 58a configures the P-GW 53a to report the PLMN-ID change (S305). In addition, the P-GW 53a reports a PLMN-ID for the network (i.e., the V-PLMN) serving the UE 10 to the PCRF 58a on the basis of information obtained in the PDN setup procedure (S307). As the PLMN-ID change notification is subscribed for the first time, the PCRF 58a reports the PLMN-ID for the V-PLMN to the P-CSCF 61a (S309).
That is, entities of the home network acquire an identifier of the visited network (or VPLMN-ID) in an IMS registration procedure. The VPLMN-ID acquired in this manner may be used in charging, roaming registration restriction, or bear creation for an additional service, or the like.
The P-CSCF 61a adds the PLMN-ID to a P-visited-network-ID header of the register message, and transfer to the I-CSCF 64a the register message to which the PLMN-ID is added (S 311).
In addition, a subsequent IMS registration procedure is performed in the same manner as described with reference to FIG. 8.
Meanwhile, when the network serving the UE 10 is changed due to a movement of the UE 10, the P-GW 53a of the home network may identify a change of the PLMN-ID. Upon identifying the change of the PLMN-ID, the P-GW 53a reports to the PCRF 58a an event occurrence based on the PLMN-ID change. Upon receiving the report of the event occurrence based on the PLMN-ID change, the PCRF 58a reports a new PLMN-ID to the P-CSCF 61a. 
However, in order for IMS entities of a home network to acquire a PLMN-ID of a visited network in a state where the UE is roaming based on the HR scheme, as described above, it takes a long time to create an Rx interface by receiving a register message from the UE, to register a PLMN-ID change notification to the PCRF, and to receive a report for the PLMN-ID.
Therefore, there is a need for a solution which allows the IMS entities to more effectively acquire the PLMN-ID of the visited network in the state where the UE is roaming based on the HR scheme.